A typical receiver 100 is composed of three general elements, as shown in FIG. 1. These elements are: 1) the radio frequency (RF) section 102 (often implemented with a chipset) which converts the RF signal into an intermediate frequency (IF), or baseband signal; 2) the analog-to-digital converter (ADC) 104 to digitize the signal; and 3) the digital portion 106 of the receiver 100 that implements receiver processing algorithms, such as channel estimation, equalization, decoding, signal-to-interference-plus-noise-ratio (SINR) estimation and automatic gain control (AGC).
Each element 102, 104, and 106 is configured so that an overall maximum carrier-to-interference ratio (C/I) requirement (e.g., 33 dB) is achieved when all the communication impairments are taken into account. Here, C refers to the desired signal power and I is the interference power due to all the hardware impairments. The maximum C/I requirement is chosen to achieve some adequate level of performance at the most aggressive modulation and coding scheme (MCS), such as R=¾ 64-QAM (Quadrature Amplitude Modulation), under certain channel conditions.
A conventional automatic gain control (AGC) algorithm tries to maintain a constant average power into the ADC 104 while using a gain distribution in the RF line-up that maximizes the overall C/I performance. This constant average power level is referred to as the “target” level. The conventional AGC, shown as a partial block schematic diagram in FIG. 10, maintains this constant power by periodically measuring the received signal strength (RSS) of the downlink (DL) at times where the DL signal is present and has reliable and consistent power characteristics. These measurements are averaged over time and the result is used to periodically update the receiver gain settings (usually once per frame). For example, in a system which implements the OFDMA physical layer in the IEEE 802.16e standard, the base station always transmits the preamble at a constant power so the preamble can be used for AGC purposes.
The conventional AGC works well when the power measurements are made over a signal that is transmitted in a manner known by the receiver. However, some systems employ multiple transmission techniques in a single frame, some of which are not known at the receiver. Some of these techniques vary the power level received at the mobile device. Additional effort must be taken at the mobile device to adequately accommodate such signals. For example, in an 802.16e system that employs downlink (DL) closed loop transmit beamforming (TxAA), the mobile device's AGC 108 sets the receiver gain based on the preamble transmission, which is the only part of the DL portion of the frame that is guaranteed to be present. When instructed, the mobile device transmits a sounding waveform to the base station at the end of the uplink (UL) portion of the frame. The base station estimates the channel from the received sounding waveform and uses the result to formulate the appropriate antenna weights to “beamform” the next DL data transmission to that mobile device.
Since the channel changes over time, the beamformed transmission must happen as soon as possible after the sounding transmission for optimal performance. Therefore, the best place to put the TxAA “zone” is right after the UL and DL maps. If a conventional AGC is used and the mobile is given a TxAA allocation that spans the entire frequency range of the channel, the mobile will see an IF (or baseband) power rise of approximately 10 log (NANT) dB in the TxAA portion of the frame, where NANT is the number of elements in the base antenna array. If only a portion of the bandwidth is allocated to the mobile device, then the power rise may not be as great as that given in this equation. For an eight-element base antenna array, the power rise can be as much as 9 dB. However, if this power rise is not accounted for, the DL performance will be degraded by clipping within the ADC 104 and added inter-modulation distortion from the RF portion of the receiver. In this case, applying TxAA techniques will degrade system performance instead of improving it. This point is illustrated by the following formulas:I=IOC+IIM+IRF+N+Q SIF=NUSED(C+I−Q)where:                I is the total interference on the digital signal as seen in one subcarrier;        IOC is the interference from other cells as seen in one subcarrier;        IIM is the inter-modulation interference from the RF receiver (which is a function of the total RF signal power (SRF) and the total gain in the receiver (G)) as seen in one subcarrier;        IRF is the interference from other RF impairments such as phase noise and reference spurs (as seen in one subcarrier);        Q is the ADC 104 quantization noise in one subcarrier;        N is the thermal noise seen at IF in one subcarrier;        NUSED is the number of subcarriers that are used;        SIF is the total IF signal power; and        C is the desired signal power at IF in one subcarrier.The signal-to-interference-plus-noise ratio (SINR) per subcarrier of the digitized signal is SINR=C/I.        
FIG. 2 shows an example of the conventional AGC operating in an 802.16e system where DL beamforming techniques are employed during part of the frame. This is a strong signal case where the performance is limited by interference from other cells, Ioc, rather than the thermal noise, N. IRF is not included here because the inter-modulation distortion, IIM, is expected to be the dominant factor. This figure shows the relative power levels versus time of the various signal components as seen by the ADC 104. As per the 802.16e specification, the occupied preamble subcarriers are boosted by 9 dB as indicated by the initial level of C. Since every other cell is also boosting their preamble power, Ioc is increased by 9 dB during that portion of the frame. After the preamble, C and Ioc drop by 9 dB for the map portion of the frame. However, since only ⅓ of the preamble subcarriers are used, SIF drops by only ˜4 dB and, thus, the inter-modulation, IIM, drops by 12 dB. During the TxAA portion of the frame, C increases by a 9 dB beamforming gain giving the desired increase in signal power but IIM increases by 27 dB. After the TxAA portion of the frame, C drops by 9 dB and IIM drops by 27 dB. The total interference, I, is shown in the figure as a dotted line. For clarity, the dotted line is slightly separated from the other curves in both time and amplitude. The vertical separation between the C and I curves indicates the SINR that the mobile sees. As shown in the figure, there is no SINR benefit for the TxAA portion of the frame over the non-TxAA portion of the frame. To make matters worse, the increased level of SIF during the TxAA portion of the frame reduces the ADC 104 headroom that is required to handle the signal's peak-to-average power ratio (PAPR), which means the signal will clip much more often. This will produce performance degradation due to the resultant signal distortion.
Therefore, a need exists to overcome the problems with the prior art as discussed above.